1. Field of the Invention
The present invention relates to a motor speed control device for use in an image forming apparatus like an electrophotographic copying apparatus or a laser printer for example.
2. Description of the Prior Art
Taking an electrophotographic copying apparatus by way of a typical example of conventional image forming apparatus, an analogue-format proportional intergration control device, a digital servo system introducing software as described in the Japanese Laid-Open Patent Publication No. 58-76909 of 1983, or an analogue PLL system is made available for controlling the speed of DC motors designed to drive a light-exposure optical system for an original to be copied, a photosensitive drum, and fixing rollers.
On the other hand, there is a tendency that some of electrophotographic copying apparatuses available today incorporates multiple functions including zooming function, double-face copying function, masking function, trimming function, automatic selection of copying paper, detection and designation of jammed paper position, or the like. In consequence, conventional motor speed control device unavoidably involves complexity.
FIG. 1 shows an example of the structure of the analogue PLL control circuit as one of the conventional motor speed control systems for an image forming apparatus.
This conventional analogue PLL control circuit is composed of the following; a speed feedback circuit 200 which compares an encoder signal with a speed instruction signal delivered from an external source, and then amplifies the difference, where the encoder signal is generated by an encoder 101 which detects the number of rotation of the DC motors built in the image forming apparatus; a phase feedback circuit 201 which calculates the differential phase and amplifies this data signal by comparing the reference clock pulse with the encoder signal a gain setting circuit 202 which adds up the differential speed signal and the differential phase signal before controlling current flowing into the DC motors; a regulator 203; and a current amplifier 204.
As is clear from FIG. 1, each of these circuits mentioned above incorporates an operational amplifier and a number of resistors and capacitors. These circuit elements are easily subject to adverse influence of external interference caused by varied ambient temperature and humidity, varied power voltage, or noise, for example. Furthermore, in order to compensate for uneven performance characteristic of sensors and those circuit elements, extremely fine adjustment must be executed by the regulator 203 while the assembly process is under way. This is turn incurs much inconvenience to all the concerned. And yet, there is a certain limit in consummating the fine adjustment. Furthermore, provision of a large number of circuit components obliges the manufacturer to incur the increased cost. In addition, provision of substantial inner space for these components results in the expanded dimension of the system, and yet, lowers reliability.
To solve those problems mentioned above, a digital servo control system introducing software and being free from external interference was previously proposed by the Japanese Laid-Open Patent Publication No. 58-76909 of 1983 as cited earlier. An example of this digital servo control system is shown in FIG. 2. This system introduces a central processing unit CPU 300, a read-only memory ROM 301, and a random access memory RAM 302. Based on an encoder signal designating the number of the rotation of a DC motor 100 detected by an encoder 101, the CPU 300 outputs a motor speed control signal for delivery to a PWM converter 303, which then converts this control signal into a pulse-width modulated signal. The pulse-width modulated signal is then amplified by an amplifier 304 before the amplified signal starts to control the speed of the rotation of a DC motor 100.
On the other hand, since the above digital servo control system needs to operate a CPU, when providing solely usable CPUs for all the built-in DC motors, it in turn results in the increased cost. To solve this problem, a single CPU which is normally used for the sequence control is made available for concurrently controlling the speed of the rotation of each DC motor built in the image forming apparatus. This effectively saves cost. On the other hand, since the operational sequence of any electrophotographic copying apparatus must be controlled with an extremely fast speed, part of the sequential control processes must be deleted, or alternatively, the copying apparatus is obliged to sacrifice the motor control performance characteristic, thus raising another critical problem.